Coverage-dependent adsorption behavior of benzene on Si(100): A high-resolution photoemission study

The adsorption of benzene molecules on Si(100) is investigated by high-resolution core-level photoelectron spectroscopy using synchrotron radiation as well as ultraviolet photoelectron spectroscopy (UPS) for valence bands. The C $1s$ photoelectron spectra of benzene adsorbates are found to consist of two different components, which represent carbon atoms with or without a direct Si-C bond. The population ratio of these two components exhibits a drastic change upon increasing the molecular coverage. This is explained by the coverage-dependent change of the adsorbate structure, which is corroborated by the noticeable variation of the molecular orbitals in the valence bands as observed by UPS. From these results, it is deduced that the population of the di-σ structure increases significantly at a high coverage over the tetra-σ structure which is energetically favored at a low coverage. This indicates the importance of the interaction between the adsorbate molecules at a high coverage.